Add documentation for rfl::Commented feature #509

Job	Run time
windows-msvc-shared (CBOR)	8m 51s
windows-msvc-shared (JSON)	11m 20s
windows-msvc-shared (MSGPACK)	3m 42s
windows-msvc-shared (UBJSON)	8m 6s
windows-msvc-shared (TOML)	3m 55s
windows-msvc-shared (XML)	3m 53s
windows-msvc-shared (FLEXBUFFERS)	6m 14s
windows-msvc (XML)	3m 46s
windows-msvc (CAPNPROTO)	8m 7s
windows-msvc (MSGPACK)	3m 35s
windows-msvc (JSON)	11m 12s
windows-msvc (benchmarks)	47m 36s
windows-msvc (CBOR)	8m 53s
windows-msvc (AVRO)	7m 54s
windows-msvc (FLEXBUFFERS)	5m 38s
windows-msvc (UBJSON)	6m 44s
windows-msvc (PARQUET)	40m 3s
windows-msvc (TOML)	3m 57s
windows-msvc-shared (PARQUET)	38m 56s
windows-msvc (YAML)	4m 34s
windows-msvc-shared (YAML)	4m 54s
windows-msvc-shared (CAPNPROTO)	7m 49s
windows-msvc-shared (AVRO)	7m 2s
	4h 16m 41s

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